Recently, an excimer laser or a UV DPSS (Diode Pumped Solid State) laser that is a high power UV laser is used in an LLO (Laser Lift Off) process that separates a thin film on a substrate using a laser beam in order to manufacture small devices such as an LED (Light Emitting Diode). Further, an excimer laser or a DPSS laser is used in a process of depositing an amorphous silicon layer and then crystallizing it in order to manufacture an LTPS (Low Temperature Poly Silicon) thin film transistor that is used as a switching device in liquid crystal panels.
In order to ensure a tightly focused and highly resolved beam spot in a process requiring high energy, it is required to reduce the size of the beam spot. Further, the intensity of a beam should be uniformly distributed in the beam for uniform processing.
There has been disclosed in U.S. Pat. No. 7,286,308 a method of exchanging beam qualities of a long axis and a short axis using a prism array to focus light from a laser diode bar. According to the method of U.S. Pat. No. 7,286,308, when a prism array is used, it is difficult to manufacture a precise optical system due to complicated manufacturing, assembling, and arranging, and the size of an optical system is too large, so practicability is low.
Further, U.S. Pat. No. 5,513,201 employs a device that rotates a light path to focus light to an optical fiber in a laser diode array system and proposes a method that uses a lens, a prism, and a mirror to rotate a light path. However, it is still difficult to manufacture a precise optical system due to complicated manufacturing, assembling, and arranging, and the problem that the size of an optical system is too large still remains. In particular, when a cylindrical lens array is used, there is a problem that the larger the focal length, the larger the beam loss. This leads to a focal length being small which causes an optical aberration.
Further, a system that requires a power threshold over a predetermined level has a limit in the maximum output of laser light, so it is required to combine two or more lasers in some cases. Accordingly, there is a need for an optical system that makes a flat top line beam profile that can perform uniform processing throughout the entire area at a time by combing two or more lasers. However, the method disclosed in U.S. Pat. Nos. 7,286,308 or 5,513,201 needs a separate optical system because it is difficult to combine two or more lasers in one optical system.
Further, it is required to construct an optical system that maintains a small spot size in the direction of a short axis (y axis) and has a flat-top profile not only in the direction of a long axis (x axis), but the short axis (y axis) in order to generate a high-density energy line beam and uniformly distribute the intensity of the beam.